1. Field of Invention
This invention is in the field of radar signal processing and describes a method of combining information derived from a plurality of sub-pulses to increase apparent radar bandwidth.
2. Description of the Related Art
Synthetic Aperture Radar (SAR) radar is used for ground mapping as well as target identification. The general principle behind SAR is to coherently combine the amplitude and phase information of radar returns from a plurality of sequentially transmitted pulses from a relatively small antenna on a moving platform.
The plurality of returns generated by the transmitted pulses along a known path of the platform make up an array length. During the array length, amplitude as well as phase information returned from each of the pulses, for each of many range bins, is preserved. The SAR image is formed from the coherent combination of the amplitude and phase of return(s) within each range bin, motion compensated for spatial displacement of the moving platform during the acquisition of the returns for the duration of the array length.
The plurality of pulses transmitted during an SAR array length, when coherently combined and processed, result in image quality comparable to a longer antenna, corresponding approximately to the xe2x80x9clengthxe2x80x9d traveled by the antenna during the array length.
Range target resolution in SAR images is determined by the radar bandwidth. Range resolution is inversely proportional to radar bandwidth. That is, the higher the bandwidth, the smaller the details of a radar scatterer can be discerned in a range bin. Therefore, ideally, radar imaging is best enhanced by short pulses of high peak power. However, because of various cost and engineering constraints, only relatively low peak power, longer transmitted pulses are generally available. To avoid this high peak power limitation, sometimes encoded, stretched pulses are used. One example of coding of longer transmitted pulses is linear phase modulation (LFM)xe2x80x94also known as a chirp. LFM coding is used to transmit sub-pulses that have varying center frequencies with limited bandwidths and combine the returns for each sub-pulse thereby increasing the effective bandwidth for imaging. This technique of combining the bandwidth of the radar by combining multiple stepped frequency pulses is the step stretch or stepped frequency method.
In the prior art, step stretch methods have combined xe2x80x9cchirpedxe2x80x9d pulses in typical de-chirp processing. However, the full potential of combining returns of chirped sub-pulses has not been reached. Thus, the present invention details combining signals for post de-chirp processing and A/D conversion.
By combining reflected radar returns from two or more sub-pulses, a radar system has improved range resolution. In the case of two sub-pulses, linear frequency modulated first radar returns reflected from a transmitted first sub-pulse and second radar returns reflected from a transmitted second sub-pulse form an image generated from said returns. Said first transmitted sub-pulse has linear frequency modulation centered about a first center frequency, f1, and is transmitted at time t1. Said second transmitted sub-pulse having linear frequency modulation is centered about a second center frequency, f2, and transmitted at time t2. The second center frequency is typically higher than said first center frequency. The first transmitted sub-pulse and the second transmitted sub-pulse have a linear frequency modulated chirp slope xcex3. The radar comprises means for sample shifting and phase adjusting said first radar returns reflected from said transmitted first sub-pulse with respect to said second radar returns reflected from said second pulse to form a line of frequency modulated chirp slope xcex3 with respect to time, said line connecting said first center frequency with said second center frequency.
Said first sub pulse and second sub pulse typically have equal time duration, where the first and the second center frequency are equidistant from a reference frequency. The returns are reflected by a target located at a location near a reference point s. The radar has means for computing said reference frequency fref centered with respect to said first center frequency and said second center frequency,             f      ref        =                            f          1                +                  f          2                    2        ;
a reference time             t      ref        =                            t          1                +                  t          2                    2        ;
a time delay xcfx84s,m to said reference point s with respect to time tm for m=1,2;
a time delay xcfx84s,ref to said reference point s with respect to said reference time tref where             τ              s        ,        ref              =                            τ                      s            ,            1                          +                  τ                      s            ,            2                              2        ;
Also provided is means for time shifting said first sub pulse returns received from said first sub-pulse by an amount       Δ    ⁢          xe2x80x83        ⁢          τ      m        =            -                        (                                    f              ref                        -                          f              m                                )                γ              +          τ              s        ,        ref              -          τ              s        ,        m            
thereby time shifting and phase adjusting said return information obtained from said first sub-pulse with information obtained from said second sub-pulse to increase the apparent receiving bandwidth of said radar. The second sub-pulse is similarly shifted.
The radar uses the first sub-pulse having linear frequency modulation extending from a minimum frequency of f1,min to a maximum frequency f1,max. The second sub-pulse has linear frequency modulation extending from a minimum frequency of f2,min to a maximum frequency f2,max, f1,max and f2,min can be equal, but typically overlap. Data generated within this overlap is either deleted so as not to contribute to the resulting radar image of the target, or is merged or concatenated for magnitude and phase continuation. The result is a pulse return that effectively equals in bandwidth that formed with a single pulse transmission that has the sum of the non-overlapping sub-pulse bandwidths, thus improving image resolution.
As an example, consider a radar that has a target resolution xcex4, number of steps to form a full pulse (number of sub-pulses) Mstep, main lobe broadening factor k, frequency overlap (normalized by the sub-pulse bandwidth) O"ugr" and speed of light C. Then, the radar full bandwidth is       BW    =          kC              2        ⁢                  xe2x80x83                ⁢        δ              ,
the sub-pulse step bandwidth BW1 is             BW      1        =          BW                                    M            step                    ⁢                      xe2x80x83                    ⁢                      (                          1              -                              O                v                                      )                          +                  O          v                      ,
and a center frequency fc1(m) at step m for m=1, 2 . . . Mstep is computed from             f      c1        ⁢          xe2x80x83        ⁢          (      m      )        =            f      c        -          BW      2        +                            BW          1                ⁡                  [                                                    (                                  m                  -                  1                                )                            ⁢                              xe2x80x83                            ⁢                              (                                  1                  -                                      O                    v                                                  )                                      +                          1              /              2                                ]                    .      
Typically, the radar de-chirps first radar returns reflected from said transmitted first sub-pulse and second radar returns reflected from said transmitted second sub-pulse, and so on, prior to sample shifting and phase adjusting.
In one example, the radar de-skews first radar returns reflected from said transmitted first sub-pulse and second radar returns reflected from said transmitted second sub-pulse, and so on, prior to said sample shifting and phase adjusting.
In another example, the radar de-skews the first radar returns reflected from said transmitted first sub-pulse and second radar returns reflected from said transmitted second sub-pulse, and so on, after said sample shifting and phase adjusting.